Pyrotechnic gas generators for motor vehicle safety use pyrotechnic charges constituted by propellant having a combustion speed (or burn rate) that depends on operating pressure.
Thus, below a certain pressure, the propellant goes out, and above its nominal pressure range, its combustion performance changes rapidly.
Solutions have already been proposed to solve that problem.
Thus, U.S. Pat. No. 5,700,030 describes a gas generator for a motor vehicle safety device, the generator comprising a first combustion chamber containing a pyrotechnic initiator and a pyrotechnic charge whose combustion is triggered by said initiator, and a second chamber for diffusion purposes, into which the gas that results from the combustion of said charge is transferred prior to being exhausted to the outside.
Those two chambers are separated from each other by a partition that presents a communication orifice that is closed by a capsule.
The diffusion chamber is provided with a piston suitable for closing or not closing said orifice as a function of the gas pressure in the chamber. The piston is urged continuously towards its orifice-closing position by a Belleville spring.
In practice, as soon as the gas pressure in the combustion chamber becomes sufficiently large, it pushes back the piston, compressing the spring.
That enables combustion of the propellant to be managed as well as possible.
Japanese patent document No. 08/156,737 describes a gas generator for an airbag, in which there are also provided means for regulating the pressure in the combustion chamber.
The combustion chamber is formed by a stationary housing having a perforated cylindrical wall and a piston, likewise constituted by a cylinder, and suitable for sliding in the housing. Depending on the position of the piston, a greater or smaller number of holes through the wall are uncovered, thereby putting the combustion chamber into communication with the diffusion chamber.
In certain embodiments, the piston is mounted on a spring. In another variant, a mass of elastically deformable material allows the piston to slide reversibly.
U.S. Pat. No. 6,062,598 describes a hybrid generator having a reserve of gas, which generator further includes a valve placed transversely between the combustion and diffusion chambers. The function of the valve is to constrict a channel interconnecting the two chambers.
The valve comprises a piston secured to a temperature-sensitive metal spring type element.
Depending on the position of the piston, the section available in the channel varies as a function of temperature in order to compensate for the temperature-associated pressure differences in the gas supply.
Nevertheless, it is clearly stated that the channel between the two chambers is never closed off completely. It is also stated that the position of the valve remains stationary while the generator is in operation. The valve is moved solely by the metal element that is sensitive to temperature.
The temperature range within which the generator is required to operate typically extends from −35° C. to +85° C.
The speed of combustion of the propellant also depends on temperature, which means that with a flow-rate control section that is fixed, the pressure in the combustion chamber increases with temperature, thereby leading to differences in performance depending on temperature.
The system described in above-cited U.S. Pat. No. 5,700,030 and JP-A-08/156,737 do not enable the problem to be solved since the means that they contain operate independently of temperature.
As for the system described in U.S. Pat. No. 6,062,598, its operation is strictly a function of temperature and does not take account of the pressure in the combustion chamber.
An essential aim of the present invention is to provide a gas generator whose operation is regulated, while taking account both of the pressure in the combustion chamber and of the outside temperature.
Thus, the invention mainly provides a gas generator for a motor vehicle safety device, the gas generator comprising:
a first chamber for combustion containing a pyrotechnic initiator and a pyrotechnic charge whose combustion is triggered by said initiator; and
a second chamber for diffusion into which the gas that results from combustion of said charge is transferred prior to being exhausted to the outside;
these two chambers being separated from each other by a “nozzle” partition presenting at least one communication orifice.
According to the invention, said diffusion chamber is provided with a piston suitable for closing or not closing said orifice as a function of the gas pressure in the combustion chamber, said piston being urged to its position for closing the orifice by a mass of elastically deformable material, said mass presenting stiffness that varies as a function of temperature, said stiffness decreasing with increasing temperature.
By means of this characteristic, if the outside temperature is low, then the stiffness of the mass increases, thereby reducing the flow section for gas through the orifice, and possibly increasing the pressure in the combustion chamber.
Conversely, if the temperature is high, stiffness decreases, thereby increasing the flow section and reducing the pressure in the combustion chamber.
This ensures that pressure is regulated as well as possible.
According to other characteristics of the generator that are advantageous but not limiting:
said mass of elastically deformable material is made of elastomer;
said mass is constituted by a cylindrical pad;
when said generator presents a shape that is essentially cylindrical, said orifice and said piston extend on its longitudinal axis; and
the proximal end of the piston for closing said orifice presents a profile that is frustoconical.
Finally, the invention also provides a motor vehicle safety device such as an airbag trigger or a safety belt retractor, that is fitted with a generator in accordance with any of the above-presented characteristics.